You forget something significant about the next Xbox. It'll come shipped with the Kinect 2. And they want ALL of that (Box + Controller + Kinect) to sell for approximately $399 and still get them a profit, or at least break even.
Nintendo, selling a WiiU at $349, actually gets a slight loss (they said they break even on one software sale). Why? It has far more meager hardware than that Newegg PC you posted, after all. It's the controller with a touchscreen that is driving it up. Xbox Next will have a similar problem.
I'd like a 7770 or even 7750, but there's a reason the rumour of 6670 came around. Gameplay and features (or gimmicks, your choice) is trumping graphics this time around. All sides learned from the Wii. They realize they need a graphic improvement too, because the Wii also taught the failures of pathetic hardware running modern games, but even a 6670 vaults a new console decently ahead of the 360 and PS3 - something you couldn't say about the Wii compared to the Xbox 1 and even the Gamecube.
It's just that a 6670 is pathetic compared to our current rigs. I find it rather saddening that a MACINTOSH from nearly a YEAR BEFORE the Xbox 720 and PS4 launch has a far superior graphics chip in it. Seriously, the new iMac packs a beastly GTX 680M or even an option for the juggernaut 680MX, the latter actually trading blows with the desktop GTX 660 Ti. Thank god PC gaming has a strong forecast for once (compare 2012 to 2010's "PC gaming is dead").